Seattle Genetics Reports Continued Advances with SGN-70 and Its ADC Technology at the AACR Annual Meeting.

BOTHELL, Wash. -- Seattle Genetics, Inc. (Nasdaq:SGEN) announced today that it is reporting data on its SGN-70 program and proprietary antibody-drug conjugate (ADC) technology during six poster presentations at the 2006 Annual Meeting of the American Association for Cancer Research (AACR). The meeting is being held April 1-5, 2006, in Washington, DC.

"We are pleased to report strong preclinical data at AACR on our SGN-70 product candidate, a humanized anti-CD70 monoclonal antibody that we expect to enter clinical trials in 2007," said Clay B. Siegall, Ph.D., President and Chief Executive Officer at Seattle Genetics. "In addition, we are reporting continued progress with our ADC technology, including key advances with our stable linker systems and potent, synthetic drug payloads that can increase both the efficacy and therapeutic window of targeted cancer therapies. We are planning to file an IND for our SGN-35 ADC product candidate in mid-2006, and several of our collaborators are also advancing toward clinical trials with products employing our ADC technology."

SGN-70 Program

SGN-70 is a humanized monoclonal antibody with intrinsic cell-killing ability. Seattle Genetics has demonstrated that SGN-70 possesses potent antitumor activity in preclinical models of both hematologic malignancies and carcinomas, including complement-dependent cytotoxicity, antibody-dependent cellular phagocytosis and antibody-dependent cellular cytotoxicity. The company anticipates filing an investigational new drug (IND) application for SGN-70 in 2007. (Abstract #3732)

Antibody-Drug Conjugate (ADC) Technology

ADCs are monoclonal antibodies linked to potent cell-killing drugs. Due to the targeting ability of monoclonal antibodies, ADCs are capable of delivering a cell-killing drug directly to tumor tissue, while minimizing toxicity to normal cells.

--Seattle Genetics is developing highly potent auristatin derivatives, including monomethyl auristatin E (MMAE) and monomethyl auristatin F (MMAF), as the drug payloads for its ADC technology. The company is reporting data describing the rate of ADC internalization, the precise chemical structures and rates of release of the drugs from internalized ADCs, and the resulting high concentrations of drugs delivered inside of target cells. (Abstracts #1982 and #1992)

--The proprietary linker systems employed in Seattle Genetics' ADC technology are designed to be stable in the bloodstream, but to conditionally release the drug payload from the antibody upon internalization to target cells, resulting in cell death. The company is reporting data on its linker systems that demonstrate enhanced stability in preclinical models. (Abstract #1991)

--SGN-75, an anti-CD70 ADC, is being developed for the treatment of renal cell cancer (RCC) and hematologic malignancies. The company is reporting preclinical data demonstrating the high expression of CD70 on primary RCC tissue, as well as the in vitro potency of SGN-75 against RCC. (Abstract #3733)

--Seattle Genetics is also developing monoclonal antibody fragments for drug conjugation. Preclinical studies demonstrate that antibody fragment-drug conjugates retain the antigen binding and antitumor activity of conventional ADCs. The potential advantages of such conjugates include rapid clearance from the body and enhanced tumor penetration. (Abstract #1189)

About Seattle Genetics

Seattle Genetics is a biotechnology company focused on the development of monoclonal antibody-based therapies for the treatment of cancer and immunologic diseases. The company is conducting multiple clinical trials of its three lead product candidates, SGN-30, SGN-40 and SGN-33, and preclinical development of several late-stage programs. In addition, Seattle Genetics has developed proprietary antibody-drug conjugate (ADC) technology comprised of highly potent synthetic drugs and stable linkers for attaching the drugs to monoclonal antibodies. The company currently has license agreements for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including Genentech, Bayer, CuraGen and MedImmune. More information about Seattle Genetics' pipeline and technologies can be found at www.seattlegenetics.com.

Certain of the statements made in this press release are forward-looking, such as those, among others, relating to the potential therapeutic potential of Seattle Genetics' product candidates and ADC technology. Actual results or developments may differ materially from those projected or implied in these forward-looking statements. Factors that may cause such a difference include risks related to adverse clinical results as our product candidates move into and advance in clinical trials, risks inherent in early stage development and failure by Seattle Genetics to secure or maintain relationships with collaborators. More information about the risks and uncertainties faced by Seattle Genetics is contained in the Company's filings with the Securities and Exchange Commission. Seattle Genetics disclaims any intention or obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.